Based on original code by Christophe Gisquet in 2010, updated to work
with current ffmpeg APIs.

Supports grabbing a single window or an area of the screen, including
support for multiple monitors (Windows does funky stuff with negative
coordinates here).

I've moved most of the configuration to AVOptions; the input file name
is now only the string "desktop", or "title=<windowname>" to select a
single window. The AVOptions are the same as x11grab where possible.

Code has been added to support a "show_region" mode, like x11grab, which
will draw a rectangle on the screen around the area being captured.

Instead of duplicating code for paletted image handling, I make use of
the GDI API's ability to output DIB (BMP) images, which can be run
through ffmpeg's existing BMP decoder.